Fading occurs when radio signals propagate along direct and indirect paths to the receiver. This phenomenon is caused by a variety of atmospheric conditions, such as temperature, humidity, pressure gradients, and signal reflections off buildings or other structures. The resulting interaction of direct and indirect radio signal propagation is undesirable in communication systems as it produces amplitude and delay distortion in the received signal. At times, this distortion is severe enough to render the received signal unintelligible.
While the duration of a fade typically has a statistical distribution, in mobile radio systems, which comprise a fixed base station and mobile units, the fade duration is also affected by the speed of the mobile unit. When the mobile unit is stationary or moving slowly, the average fade duration is longer than when the mobile unit is moving more rapidly. These longer fade intervals, which can each last for several hundreds of milliseconds, are particularly troublesome as it is more difficult, if not impossible, for receiver curcuitry to regenerate the information transmitted after such long periods of distortion. In addition, in some communication systems an established channel between the base and mobile unit may be dropped once supervision is lost for a prescribed time interval. These problems have led to the development of a variety of fade compensating techniques.
In one technique applied to digital signals, error correction schemes are utilized to reconstruct the originally transmitted signal. The problem with this technique is that the error correction schemes required for long fade intervals become rather unwieldy and produce excessive signal delays.
In another technique known as switched space diversity, a number of antennas are used at the transmitter or receiver along with apparatus which selects the optimal antenna at a particular time. While this technique is appropriate for digital and analog telecommunications applications, the abrupt switching of the information signal from one antenna to another at the transmitter increases the signal bandwidth, produces spurious emissions, and requires a feedback control path from the receiver to the transmitter. Moreover, abrupt switching of the incoming information signal from one antenna to another at the receiver produces undesirable interference. Switched space diversity is also inappropriate for certain systems applications, e.g., mobile radio systems, because the required use of multiple antennas in a mobile unit is cumbersome and expensive.
In still another technique known as maximal ratio space diversity or selection space diversity, multiple receivers are utilized along with apparatus which weighs or selects the receiver output signals. The shortcoming of this scheme is that the cost of the necessary multiple receivers can exceed system objectives.